The intrinsically conductive polymers (ICPs) owe their electrical conductivity to the presence of a π electronic system of conjugated type in their structure1-3. Polyaniline (PANI) is an ICP which shows relatively low conductivity values compared to other ICPs such as doped polyacetylene or polyphenylene, but has the advantage of presenting high stability and workability4. Due to its easy preparation, good ambient stability, interesting redox properties and potential for applications in electronic and optical devices5, polyaniline has become one of the most important conductive polymers and has been intensely studied over the last two decades6-7. In a recent study8 it was demonstrated that polyaniline can be used for the synthesis of functional inks which can be used for digital printing and deposited as thin films on flexible polymer substrates; polyaniline also shows particular electronic properties, such as negative overcapacity in the low frequency range. The particular trend of the charge vs. DC voltage diagram, in a “butterfly” form, is clear evidence of the underlying memristive, or better “memcapacitive” properties, as suggested in Di Ventra et al.9. Promising memristive properties have been found in devices produced with nanocomposites of PANI-graphene10, which provide the base for future application in neuromorphic systems11-12.
Although ten years have elapsed since its discovery13, two-dimensional graphene with honeycomb lattice continues to arouse great interest due to the non-conventional phenomena associated with this material, mainly originating from the fact that the electrons belonging to the carbon atoms are obliged to move in the plane defined by the graphene sheet according to paths with hexagonal symmetry, a fact which makes them similar to massless particles14. The introduction of graphene in PANI matrices to form nanocomposite structures of PANI/graphene has led to an improvement in the electronic characteristics of the ICP and also to interesting new electronic effects. In particular, in accordance with the quanto-relativistic effects in the scattering of electrons due to short-radius disturbance potentials such as those introduced by charged groups into the adjacent ICP matrix, the massless Dirac fermions which interfere with small scattering centres make a negligible contribution to the overall resistivity. Only in the case of resonant scattering can it be found that the contribution to the resistivity is equal to that of a non-relativistic electron gas15.
Due to the presence of oxygen functions on the basal planes and on the edges, which make them very hydrophilic, graphene oxide sheets (GO) are often preferred to graphene in the formulation of polymer composites, since in water they swell up and are easily dispersed16.
Furthermore, it has been recognised for some time that GO has strong oxidising properties17 and it has therefore been used in the one-pot synthesis of PANI/graphene composites, coupling the GO reduction process with the oxidative polymerisation process of the aniline precursor18. However, this process has a limiting phase consisting in dimerization of the aniline precursor19.
A known method of synthesising PANI/reduced graphene oxide composites uses a process in which a dispersion of graphene oxide in the presence of an anionic emulsifying agent is mixed with a solution of an aniline oligomer in an organic solvent.
However, the composite obtained has the drawback of resulting only in a well-defined and limited quantity of PANI formed in a stoichiometric ratio with respect to the reduction of graphene oxide.
The need is therefore felt in the art for an alternative method for the preparation of polyaniline and reduced graphene oxide composites starting from low toxicity reagents, which is conducted in an aqueous medium, which is quicker and simpler than the known methods, which provides a highly regular product and allows the preparation of stable reduced graphene polyaniline solutions with polyaniline/reduced graphene oxide ratio that can be varied as required for applications such as flexible electronics obtained by inkjet printing, and improved properties for application in electronic devices.